1. Field of the Invention
The invention relates generally to optical fibers. More specifically, the invention relates to a large mode area fiber that is formed in the shape of a helix to cause the optical fiber to exhibit a refractive index profile that minimizes bend-induced limitations on the fiber's filtering properties.
2. Discussion of the Related Art
In many optical systems and more particularly, in the field of optical fiber-based technology, there is an increasing interest in the use of large mode area fibers in which it is desirable to filter or suppress certain wavelengths of light while guiding other wavelengths of light. An important example is the filtering of light generated by stimulated Raman scattering (SRS) in propagation along an optical fiber. Light generated by SRS can act as noise in some systems, but can also lead to damage of optical components and system failure, particularly in high-power systems.
Suppressing wavelengths associated with SRS has been demonstrated to significantly improve the performance of optical amplifiers and lasers. Some amplifiers may also require suppression of amplified spontaneous emission or spurious lasing at one wavelength (e.g. 1060 nm) in order to provide efficient amplification of a wavelength with competing or intrinsically lower gain (e.g. 940 nm). Similarly, significant benefits of wavelength filtering have been demonstrated in applications such as telecommunications, sensing, etc.
It is often desirable that a fiber have a large mode area while simultaneously providing high loss of undesirable wavelengths and low loss of the signal wavelength. Filter fibers have been demonstrated, but are subject to tradeoffs. In large mode area filter fibers, there is typically a tradeoff between degree of filtering, effective area, and bend loss, e.g., a required degree of filtering becomes more difficult to achieve as mode area increases. As mode area increases, it becomes more and more difficult to simultaneously achieve low signal loss (including macro-bend and micro-bend loss), high loss of undesirable wavelengths, high yield in fiber production, and reasonable insensitivity to how the fiber is bent or arranged. Other properties (beam quality, pump absorption, birefringence, etc.) may also play an important role in overall performance, and limit the degree of filtering.
It is often desirable to implement distributed filtering along the length of an optical fiber. In one particular application of interest, a delivery fiber is employed to direct light from a source to a utilization point. It is desirable that the delivery fiber guide high power (e.g. 1 or more kW), for example, to increase speed of material removal in a machining application. It is also desirable for the delivery fiber to direct light over relatively long distances (e.g. 5-50 m), to permit maximum flexibility in the position of the source and utilization point.
Filter fibers may attain selective filtering only when bent to a particular radius of curvature and arranged in a coil of approximately that radius. This strategy does not apply directly to a delivery fiber. Arranging the delivery fiber in a coil defeats the purpose of delivering light from source to a distant utilization point, since the distance between ends of the coiled fiber is much less than the fiber length.